In a scanning image capture, e.g. in laser scanning microscopes (LSMs), conventionally two 1D scanners or one 2D scanner are used. In this case, an object of which an image is to be created is scanned point by point and line by line (scanning).
In the process, a scanning unit, for example a scanner, is placed at one end of a line to be scanned. The relevant line is scanned once or several times, and the scanning unit is placed at one end of another line to be scanned (line feed).
For the line feed, the scanning unit is moved so that during a turnaround phase of the scanning movement (subsequently also referred to as turnaround area) the scanning unit is moved perpendicularly to the direction of the rapid scanning in the direction of the line (line scan), for example one line down. Afterwards, the scanning unit remains in this direction (infeed direction) at a constant position until the line scan is completed.
Due to the limited reaction speed of a controlled scanning unit, at a higher scanning speed, the controlled scanning unit can no longer follow an intended nominal movement, which results in a position error for the line feed LF. The position error depends on the position in the rapid scanning direction and on the current direction of the line scan, e.g. forward scan or backward scan. The scanned image values result in a distorted image with additional image defects since image scanning and image representation do not match.
Such distortions and image defects are particularly problematic in bidirectional scanning given that the image errors of even-numbered and odd-numbered lines differ and lines are no longer parallel to one another. In multi-spot scans in the line direction, the line interlaces are greater, are additionally emphasized by offsetting, and are significantly more visible to the human eye due to their structure. Problematic are also strong undersampling, scanning of every x-th line and offsetting (e.g. interpolation) of the pixels between them (line-step mode) as well as resonant scans with high speed in the line direction.
One known solution for the problem is to reduce the useful area UA (FIG. 1) of each scan curve SC in order to increase the turnaround area TA given that the sum of the useful area UA and turnaround area TA is always 100%. This leaves more time to move in the direction LD of the line feed LF. By enlarging the turnaround area TA from 16% (useful area UA 84%) to 48% (useful area UA 52%), the scan can be performed three times faster; by enlarging the turnaround area TA from 16% (useful area UA 84%) to 96% (useful area UA 4%), the scan can be performed six times faster, with the same position error in the feed direction, and thus already shows the limit of the method. The useful area UA becomes increasingly smaller and converges towards zero.
In the scanning image capture, the scan is performed in the direction of the rapid scanning (line scan) with a temporally triangular scan trajectory in order to achieve a constant scanning speed over a useful area UA of a scan curve SC (FIG. 1).
In order to also compensate for remaining residual errors, a control function that is utilized for controlling the scanning unit must be pre-distorted for a scan at a constant scanning speed, as is known, for example, from U.S. Pat. No. 6,037,583.
U.S. Pat. No. 6,037,583 describes a control system for a scanner drive, in particular for a laser scanning microscope. The scanner drive includes an oscillating motor for driving an oscillating mirror used for a linearly oscillating deflection of a beam. Furthermore, a control unit for supplying the oscillating motor with an exciting current is provided, which is variable with regard to the control frequency, the frequency curve and the amplitude. A function generator is provided, which is connected to the control unit. A measuring sensor serves to obtain a sequence of information about the deflection positions of the oscillating mirror. An arithmetic unit is configured for determining correction values for the exciting current from a comparison of actual and nominal values of the deflection position. In the arithmetic unit, arithmetic circuits are provided, which are configured for converting the information about the deflection positions of the oscillation mirror according to the amplitude and phase of the scanner based on a plurality of control frequencies.
From the publication by John Giannini et al., “Driving MEMS mirrors for beyond their specification for fast, precise, synchronized laser scanning”, Focus on Microscopy 2015, a method is known, by which a pre-distorted control function for the line scan of MEMS devices is generated close to or above their resonance frequencies. To this end, a nominal function corrects and pre-distorts a control function with the deviations caused by the transmission function of the actual control function.